1. Field of the Invention
This invention relates generally to preionizers adapted for use with above atmospheric pressure gas discharge lasers and more specifically pulsed transverse electric discharge excimer or rare gas halides lasers.
2. Discussion of the Prior Art
An excimer laser functions as a result of the lasing transitions of certain noble gas-halogen molecules which normally would not exist at room temperature. The excimer molecules are typically formed in electrical discharges, where the atomic species are given sufficient energy to bond. Formation of a usable laser beam from such an electrical discharge necessitates that the discharge be as spatially uniform as possible in both laser gain (formation of upper state lasing transition) and index of refraction (depletion of discharge species).
It is well established in the field of excimer lasers that some level of ionization of the excimer gas is required prior to initiation of the main laser discharge. This preionization aids in development of the most spatially uniform electrical glow discharge. Almost all commercially available excimers utilize a transverse electrical aperture (TEA) arrangement of the discharge electrodes. This results in an electrical discharge which is narrow in the anode to cathode direction but long along the face of the electrodes. In order to keep the discharge stable along the length of the electrodes, a high degree of parallelism is required between the anode and cathode faces. This can be accomplished by constructing a main discharge electrode subassembly which mechanically supports the electrodes in relation to one another and allows the relative electrode parallelism to be established in a subassembly prior to incorporation into the laser vessel.
The preionization of the main laser discharge medium prior to initiation of the main electrical discharge, enhances the stability of the discharge thus formed. The initial electrical breakdown of the gas from a high impedance non-ionized state to a highly ionized low impedance, ideally glow discharge, requires an avalanching of the initial free electron density of the gas by many orders of magnitude. This avalanching of the electron density is accomplished under the influence of an electric field produced by a high voltage potential applied across the discharge electrodes until full discharge current conduction occurs. The generally exponential avalanche of electron density within this main discharge region is highly dependent on the spatial uniformity of the initial free electrons present and is greatly affected by the shape and position of the main discharge electrodes across which the high voltage potential is applied.
In the preionizer main discharge electrode arrangements of the prior art where this uniformity is not assured, regions of the discharge with higher initial electron number densities have tended to avalanche more quickly than weaker regions, preferentially draining the main discharge current.
Excimer lasers have typically used three major types of preionization schemes: 1) x-ray ionization, 2) corona plasma UV ionization, and 3) spark UV ionization.
X-ray preionization provides extremely uniform preionization and high levels of free electron density generation but unfortunately generally requires a separate cost-significant high-voltage driving circuit as well as a mechanical design sufficient to withstand repeated high voltage pulses in a vacuum tube environment. X-ray preionization may also find it difficult to provide a proper X-ray transparent window onto the discharge region which can mechanically withstand the force of an above-atmospheric pressure laser medium. These requirements make it difficult to implement x-ray preionization cheaply, compactly, and safely.
Corona plasma preionization involves forming a stable corona plasma in the laser gas. Corona plasma is a weak electrical discharge associated with gaseous breakdown under the influence of high electric field strength, with no current-source path through the incipient discharge. Since energy is only put into the corona plasma from the fields in the vicinity, relatively low plasma temperatures are created, producing only relatively weak UV photons. These photons do not ionize very far into the laser gas without attenuation. As a result, the corona plasma preionizer -must be located physically close to the desired laser discharge region in order to obtain enough preionization density to be significant to the laser discharge formation.
Spark preionization provides small discharge arcs along the discharge length. These arcs emit weak x-ray and hard UV photons which propagate through the discharge region. Each spark is a controlled arc-discharge with inherently high plasma temperature, producing harder UV emission than other plasma types.
The absorption of the photons produced by sparks driven by the discharge medium, may result in a decreasing preionization electron density with increasing distance from the spark. This is also true of the x-ray, and corona-type preionizers, each of which has its own absorption function based on the discharge medium. This local strong-absorption of the preionizing radiation produces a local preionization electron density. As a function of distance from the source the local preionization electron density can be measured for a particular device. This measurement can then be used to predict the preionization gradients which will be present in the main discharge region when potential is applied across the main discharge electrodes. The main discharge electrode geometry can then be suitably matched to provide the most uniform discharge.
The main drawbacks to using spark preionization are the potential for spark pin electrode deterioration, and the difficulty of implementing a uniform set of sparks along the long-axis of the laser discharge without compromising distance from the discharge or peak arc current.
In general, the energy distribution of the photons emitted from a spark can be thought of as being a function of the electric field strength across the spark pin electrodes at the time of the spark-initiating avalanche, as well as the black-body radiation distribution resulting from the temperature of the actual speck arc-discharge formed. The mechanical shape of spark pin electrodes and the driving circuit will affect the quantity and distribution of the ionizing radiation formed.